It's not like the Marlins are big crowd-pullers here anyway...only once in Nats Park history has a Marlins visit drawn over 30k (33,247 for a Strasburg start on Saturday 17th September 2011), and the poorest-drawing Saturday and Sunday matches in Nats Park history have both been Marlins visits - Saturday 11th September 2010 (17,941) and the day after, Sunday 12th September 2010 (16,788) - in fact of the 35 Marlins visits to Nats Park thus far, 14 have drawn under 20k.
Those who went to last night's match against the Astros saw only 16,245 in the stands despite Strasburg starting, in large part since last night went up against Caps-Bruins Game 3 - expect a similarly thin crowd Saturday since that day's Marlins match goes up against Game 5 of that series (face-off 15.00) which, even though it'll be in Boston this time, is at this writing a potential deciding match.
